The present invention relates to an optical system that magnifies and projects projection light that has been modulated by an image generating device such as a liquid crystal device, DMD, based on image information onto a screen, and to a rear projector that uses the same.
Japanese Laid-Open Patent Publication No. 2001-264627 discloses an optical system for a rear projector that includes an image forming optical system with a plurality of lenses and a positive power, and a projection system with a curved mirror of a negative power.
The Publication also discloses that the optical system having an image generating device that emits projection light, the lens system, and the curved mirror is disposed below a screen so as to images are displayed with the projection light incident on the screen diagonally from below. In this system, a screen (Fresnel screen) where prism-type Fresnel lenses are disposed at a narrow pitch is used. A normal Fresnel screen cannot be used when the incident angle is large due to the amount of light being insufficient. Contrary, with a Fresnel screen that uses prism-type Fresnel lenses, when the incident angle of the projection light is sufficiently large, the projection light is refracted in the desired direction and therefore bright images can be displayed. Therefore, when the incident angle is too low, the images displayed by a prism-type Fresnel screen become dark. Accordingly, combined-type screens where prism-type Fresnel lenses are disposed in a top part of the screen where the incident angle for the projected light is large are now used, but such combined-type screens are difficult to manufacture and are expensive. There is a further problem that stray light is produced due to the mixture of different types of Fresnel in a screen.
When the light is incident on the screen diagonally from below, the incident angle is smallest in a lower central region of the screen and becomes larger toward an upper part and edges of the screen. For this reason, to produce a sufficiently large incident angle in the lower central part of the screen, it is preferable to use an optical system that is a combination of a wide-angle lens system and a curved lens with negative power. However, there is the problem that when a sufficiently high incident angle is produced in the lower central part of the screen, rays are scattered in the upper part and at the edges of the screen, resulting in it becoming difficult to achieve a sufficiently high resolution. For this reason, for a slim rear projector, it is difficult to display bright, clear images across the entire screen.